Processes for the production of olefinic hydrocarbons are very useful in the production of a great many products. Because of the commercial importance of olefin compounds, those skilled in the art of producing olefins are constantly searching for better and more economical methods to produce olefins.
In U.S. Pat. No. 3,360,586 (Bloch et al), a process is disclosed for reacting a straight-chain paraffinic hydrocarbon of from about 7 to about 20 carbon atoms per molecule in contact with a Group VIII noble metal catalyst supported on a nonacidic refractory inorganic oxide carrier, hydrogen and water in an amount of at least about 400 ppm based upon the hydrocarbon.
In U.S. Pat. No. 3,907,921 (Winter), a process is described wherein the activity of a used dehydrogenation catalyst is improved by an increase in the water concentration maintained in the reactants toward the end of the catalyst's life. More specifically, the process comprises injecting 5 to 25 weight ppm of water into a hydrocarbon feed stream comprising normal paraffins having 5 to 18 carbon atoms per molecule, passing the feed in admixture with a gaseous recycle stream over a dehydrogenation catalyst comprising a platinum component and an alkali or alkaline earth component supported on a porous alumina carrier material, and increasing the rate of water injection to a value of 25 to 125 weight ppm after at least 40% of normal paraffins which may be processed before the catalyst requires replacement have passed through the reaction zone.
In U.S. Pat. No. 4,430,517 (Imai et al), a process is disclosed for the dehydrogenation of C.sub.2 -C.sub.30 hydrocarbon compounds in the presence of water injection of water in an amount of 1-20,000 wppm.
Even with the broad teaching of the prior art that the presence of water is desirable in the dehydrogenation of hydrocarbons, those skilled in the art of catalysis have believed that a platinum containing catalyst is poisoned by high levels of oxygenates by various mechanisms. Therefore, since water is an expected intermediate for the formation of oxygenates such as carbon monoxide, the addition of water or water precursors is viewed as a platinum catalyst depressant. Additionally, heavy oxygenates are thought to lead to the formation of gums or coke precursors. The presence of water in a dehydrogenation zone strips or removes chloride from the platinum catalyst to form hydrogen chloride and the water is converted to form carbon monoxide via the water shift reaction at dehydrogenation conditions. For these reasons, the introduction of water into a catalytic dehydrogenation zone is considered undesirable.
We have discovered that by introducing a relatively small amount of water or water precursor at the inlet of two or more beds of dehydrogenation catalyst, the performance of the catalyst is significantly increased without any perceptible adversity.